Deserialize a table of string values

UnpackStrings.sql

CREATE FUNCTION [dbo].[UnpackStrings]
(
     @original NVARCHAR(MAX)
    ,@delimiter NCHAR(1)
    ,@zed NCHAR(1)
    ,@escape NCHAR(1)
)
RETURNS 
@Unpacked TABLE 
(
     ElementNumber INT IDENTITY(1,1)
    ,Element NVARCHAR(MAX)
)
AS BEGIN
    DECLARE @curr NCHAR(1);
    DECLARE @next NCHAR(1);
    DECLARE @pos INT = 1;
    DECLARE @sb NVARCHAR(MAX) = '';

    -- Special characters must be distinct.
    IF ((@delimiter = @escape) OR 
        (@zed = @escape) OR 
        (@delimiter = @zed)) BEGIN
        SET @sb = CAST('Special characters must be distinct.' AS INT); -- (Can't throw or raiserror in a function)
    END

    -- Null string return a null array
    IF (@original IS NULL) BEGIN 
        SET @sb = CAST('You must check for a null-value @original prior to calling this function.' AS INT); -- (Can't return a NULL valued table in SQL)
    END

    -- A single escape character represents an array with a single 
    -- empty element to differentiate from an empty array.
    IF (@original = @escape) BEGIN
        INSERT @unpacked (Element) VALUES ('');
        RETURN;
    END

    -- Fill the table variable with the rows for your result set
    WHILE((@pos <= LEN(@original))) BEGIN
        SET @curr = SUBSTRING(@original, @pos, 1);
        IF (@curr = @zed AND (LEN(@sb) = 0 OR LEN(@sb) IS NULL)) BEGIN
            IF (COALESCE(NCHAR(UNICODE(SUBSTRING(@original, @pos + 1, 1))), @delimiter) != @delimiter) BEGIN -- Peek
                SET @sb = CAST('Bad format - ambiguous unescaped zed encountered.' AS INT); -- (Can't throw or raiserror in a function)
            END
            SET @sb = NULL;
        END ELSE IF (@curr = @delimiter) BEGIN
            INSERT @unpacked (Element) VALUES (@sb);
            SET @sb = '';
        END ELSE IF (@curr = @escape) BEGIN
            SET @pos = @pos + 1;
            IF (LEN(@original) < @pos) BEGIN
                SET @sb = CAST('Bad format - dangling escape character encountered.' AS INT);
            END ELSE BEGIN 
                SET @sb = @sb + SUBSTRING(@original, @pos, 1);
            END
        END ELSE BEGIN
            SET @sb = @sb + @curr; 
        END 
        SET @pos = @pos + 1;
    END

    INSERT @unpacked (Element) VALUES (@sb);
    RETURN
END